A $-4\mathrm{.}0{10}^{-8}C$ charge is placed $30cm$ to the right of a $2\mathrm{.}5{10}^{-8}C$ charge.

a$)$ Calculate the electric force that would be exerted on $a-1\mathrm{.}8{10}^{-9}C$ charge that is placed $10cm$ left of the $2\mathrm{.}5{10}^{-8}C$ charge. $(/8)$

b$)$ Determine the intensity of the electric field at the point where the $-1\mathrm{.}8{10}^{-9}C$ charge is placed. $(/4)$